1. Field of the Invention
The present invention generally relates to an inspection method for a magnetic recording medium provided in a magnetic recording apparatus that is used as an external storage device of a computer.
2. Description of the Related Art
Recently, information technology has been advancing and thereby it is required to increase storage capacity of magnetic recording apparatuses. In order to increase storage capacity of the magnetic recording apparatus, for example, the number of magnetic disks can be increased in a magnetic disk apparatus which is one of the magnetic recording apparatuses. However, the number of disks provided in the magnetic disk apparatus is limited because available space in the magnetic disk apparatus is limited. Therefore, it is not possible to meet a demand for increasing the recording capacity only by means of increasing the number of disks.
Therefore, it is required to increase the recording density of magnetic recording media. Conventionally, two techniques to increase the recording density of the magnetic recording media are being studied. One technique is to increase linear recording density and another technique is to increase track density.
One approach to increasing the linear recording density is to reduce magnetic spacing that is the distance between the magnetic pole surface of the magnetic head and the surface of the magnetic layer of the magnetic recording medium. The magnetic spacing is the sum of the thickness of the protection layers of the magnetic recording medium and the magnetic pole of the magnetic head and the flying height of the magnetic head. Therefore, it is required to reduce the thickness of the protection layers and the flying height of the magnetic head to reduce the magnetic spacing.
When the flying height of the magnetic head is reduced, a problem with protrusions on the magnetic recording medium appears. Therefore, an inspection is performed at the end of the manufacturing process of the magnetic recording medium to assure the smoothness of the surface of the magnetic recording medium. For example, a glide-inspection is performed for all the manufactured magnetic recording media. The glide inspection is an inspection in which relatively large protrusions on the surface of the magnetic recording medium, that might cause head crashes, are detected as defects. In the magnetic disk apparatus, the head crash means that the data write/read magnetic head comes in contact many times with relatively large protrusions on the surface of the magnetic disk and damage to the surface of the magnetic disk is expanded greatly from the protrusions, and thereby a read data error is caused, or that dust produced by the damage to the surface of the magnetic disk adheres to the flying surface of the magnetic head and therefore the stability of the flying head is lost thereby causing read data errors.
Next, the outline of the glide inspection of the magnetic disk will be explained with reference to FIGS. 1A and 1B. FIG. 1A shows a sectional diagram of an example of a magnetic disk and an example of a glide inspection head for detecting the protrusions on a surface of the magnetic disk. FIG. 1B shows an example of an output signal from a contact detection sensor on the glide inspection head.
As shown in FIG. 1A, the glide inspection head 11 is flown over the surface of the magnetic disk at a predetermined flying height from the surface of the magnetic disk 10. This flying height is referred to a flying guarantee height (a glide height 12). Attitude of the flying head 11 is such that the front part 11-1 of the flying head 11 is higher than the rear part 11-2 of the flying head 11 because air (AIR) caused by revolution of the disk flows under the flying head 11 as shown by the arrow. Therefore, the lowest point of the flying head 11 over the magnetic disk 10 is the rear part 11-2 and the distance between the rear part 11-2 of the flying head 11 and the surface of the magnetic disk 10 is the glide height 12.
If the height of the protrusion 13 on the magnetic disk 10 is higher than the glide height 12, the glide inspection head 11 comes in contact with the protrusion 13 and the shock wave propagates to a contact detection sensor 14 attached on the upper surface of a slider of the glide inspection head 11. Then, a voltage is induced in the contact detection sensor 14 and the induced voltage is the output signal supplied from the contact detection sensor 14.
FIG. 1B shows the output signal supplied from the contact detection sensor 14. The peak level 16 of the output signal greater than a predetermined threshold value 17 is detected by a signal analysis apparatus 15 connected to the contact detection sensor 14. Then, the protrusion 13 corresponding to the peak level 16 is detected as a defect.
The threshold value 17 is determined by measurement conducted as follows. First, a magnetic disk having various protrusions each of which has a diameter greater than several micrometers and the height greater than a predetermined glide height 12 is provided. Next, the magnetic disk is rotated as described in the glide inspection method. Then, when the glide inspection head 11 flying at a predetermined glide height 12 comes in contact with such protrusion, the peak level of the output signal is measured. Finally, the threshold level 17 is determined and set as a voltage that is lower than the peak level and sufficiently higher than the noise level to obtain the desired S/N ratio.
The magnetic disk having at least one such defect is usually determined to be a defective magnetic disk. These defective magnetic disks are returned for a reworking process of the surface of the magnetic disk or disposed of, and such defective disks are never integrated into the magnetic disk apparatuses. If the defective disk is integrated into the magnetic disk apparatus, the defective disk may cause a head crash.
Therefore, it is required to detect the protrusions on the magnetic disk that cause the problem to the magnetic disk apparatus by means of the glide inspection in order to reduce the flying height of the magnetic head.
On the other hand, in order to increase the track density, MR heads or spin bubble heads are employed in the magnetic recording apparatus to obtain sufficient reading output signal level when the narrow track pitch is used in the magnetic recording medium. A device part of the magnetic head consists of multi thin film layers and the protection layer is provided on the flying surface side. However, when the device part of the magnetic head comes in contact with a protrusion, the device part of the magnetic head is damaged.
A small protrusion, for example a protrusion having a small diameter, which is not detected as a defect by the conventional inspection method described above, may cause high contact pressure because the contact area is small. When the small protrusion comes in contact with the device part of the magnetic head, the device part of the magnetic head may be damaged. Furthermore, recently; the device part of the magnetic head having a width of less than 1 micrometer (μm) is being manufactured. When such a device having narrow width comes in contact with a protrusion having a diameter less than 1 μm, the device is damaged by the high contact pressure and thereby the magnetic head cannot read the signal from the magnetic recording medium.
The peak level of the output signal from the contact detection sensor caused by such a small protrusion is about a half of the conventional threshold level. Therefore, it is impossible to detect such a small protrusion by the conventional inspection method.
If the threshold level for the glide inspection is reduced to less than a half of the conventional threshold level, it may possible to detect such small protrusions. However, in this case, protrusions that are so small that the magnetic disk having such protrusions may not experience the problem even if the magnetic disk having such protrusions is integrated into the magnetic disk apparatus, may be detected as defects, or the noise signal included in the output signal from the contact detection sensor may be misdetected to be a defect. In this case, the non-defective magnetic disk is classified as a defective magnetic disk. As a result, the yield and the reliability of the glide inspection will be degraded.